1. Field of Invention
This invention relates generally to communications, and more particularly, to an digital subscriber line (xDSL) line hybrid circuit.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies"" central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: ADSL, VDSL, HDSL all of which are broadly identified as xDSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable.
Discrete Multi-Tone (DMT) is a multicarrier technique that divides the available bandwidth of twisted-pair connections into many sub channels. The DMT technique has been adopted by the ANSI TIE1.4 (ADSL) committee for use in ADSL systems. In ADSL, DMT is used to generate 250 separate 4.3125 kHz sub channels from 25.875 kHz to 1.104 MHz for downstream transmission to the end user, and 26 sub channels from 25.875 kHz to 138 kHz for upstream transmission by the end user. The transmission capability of the individual sub channels are evaluated for each connection, and data is allocated to the sub channels according to their transmission capabilities (the number of bits each sub channel can support). Sub channels that are not capable of supporting data transmission are not used, whereas the bit-carrying capacity of sub channels that can support transmission is maximized. Thus, by using DMT in an ADSL system, the transmission capability of each subscriber line is maximized over the fixed bandwidth.
Generally, the implementation of the ADSL sub channels occurs in highly integrated digital circuits which provide a low voltage, low current output signal carrying the information for each needed channel. This low level output signal is, generally, interfaced to a subscriber line through a hybrid circuit coupled to a transformer. A hybrid circuit allows real-time full-duplex data transfer. The coupling transformer is used mainly for the purpose of longitudinal signal isolation, surge protection and impedance. A hybrid circuit can provide some undesirable amount of transmission loss from the transmitting path to the receiving path. On a voice line this feedback exhibits itself as an echo or as crosstalk. For a data line these effects create an undesirable degradation of the signal integrity. For real-time full-duplex data transmission applications, the amount of this crosstalk must be minimized to insure the integrity of the data transmission.
Therefore, the need exists for a hybrid which reduces crosstalk and echo between the transmit and receive paths.
A method and apparatus for coupling an xDSL transceiver with a subscriber line is disclosed. A hybrid circuit is disclosed which exhibits a different relative gain factor for transmitted signals and received signals. The difference in gain factors results in part from a combined balance circuit and transformer which exhibit a different coupling ratio between the primary and secondary coils on the receive path and the transmit path. The hybrid circuit may be used to isolate the receive signal from the transmit signal in the full duplex mode of operation. The gain from the transmit port to the receive port is minimum and most of the energy is transmitted to the subscriber line.
In an embodiment of the invention a hybrid circuit for communicating a transmit signal and a receive signal across a subscriber line is disclosed. The hybrid circuit includes: a transformer and a balance circuit. The transformer includes a primary inductor and a secondary inductor and the secondary inductor for coupling to the subscriber line and the primary inductor including at least two inductive portions. The balance circuit includes a transmit interface to input the transmit signal and a receive interface to output the receive signal, and the balance circuit coupling both the transmit interface and the receive interface with the at least two inductive portions of the primary to change the relative magnitude of an induced emf between the primary and the secondary for the transmit signal and the receive signal.
In an alternate embodiment of the invention a method for communicating a transmit signal and a receive signal across a subscriber line is disclosed. The method for communicating comprises the acts of:
forming at least a first inductive coupling and a second inductive coupling with the subscriber line;
coupling a plurality of resistive elements to form opposing sides of a balance circuit with opposing ends, and with each of the opposing sides including a corresponding one of the first inductive coupling and the second inductive coupling;
interfacing with the opposing sides of the balance circuit to form a receive interface for outputting the receive signal and the receive interface exhibiting a first induced emf resulting from the receive signal on the subscriber line; and
interfacing with opposing ends of the balance circuit to form a transmit interface for inputting the transmit signal and the transmit interface exhibiting a second induced emf on the subscriber line resulting from the transmit signal and the first induced emf differing in magnitude relative to the second induced emf.